Maestrichtian Planktonic Foraminifer Biostratigraphy and the Cretaceous/Tertiary Boundary at Hole 738C, Kerguelen Plateau (Southern Indian Ocean)

Abstract

A biostratigraphically complete upper Maestrichtian-Danian sequence was recovered at ODP Leg 119 Site 738 on the southern Kerguelen Plateau (62°S). Planktonic foraminifers are abundant and well to moderately preserved in all upper Maestrichtian samples to within 6 cm of the Cretaceous/Tertiary boundary, where foraminifers are rare and poorly preserved due to increased sediment lithification. Lowermost Danian samples are also poorly preserved, but specimen abundance is sufficient for preliminary quantitative biostratigraphic analysis. Despite the poor foraminifer preservation, evidence is presented for paleoenvironmental change immediately below a thin, iridium-rich calcareous clay horizon that is recognized as the Cretaceous/Tertiary boundary. The most significant indicators for preboundary change include; (1) loss of sediment bioturbation; (2) loss of keeled planktonic foraminifers; and (3) diminished size and diversity of benthic foraminifers. Planktonic foraminifers showing transitional morphologies between Cretaceous and Tertiary planktonic biserial heterohelicids first appear 4 cm below the boundary clay. Although some Cretaceous planktonic foraminifers consistently occur with early Danian marker species in initially high abundance above the boundary clay, they are not considered as survival species, but are considered as reworked because of their co-occurrence with Campanian and Maestrichtian species whose extinctions took place well before the end of the Maestrichtian. The recovered Maestrichtian planktonic foraminifers are identical to Austral Realm assemblages found in the southern South Atlantic region. All five species previously recognized as being endemic to this realm were recovered from Hole 738C. Lower Danian assemblages strongly resemble coeval low-latitude foraminifer faunas, whereas upper Danian assemblages differ only in the absence of indicator species, particularly the morozovellids. The stratigraphy of this sequence is correlated using existing zonal schemes for the southern high-latitudes, with modification of one Antarctic Paleogene zone and subzone. Only broad subdivisions of tropical zonal schemes were recognized, as most of the nominate taxa are absent from the southern high-latitudes.